1. Field of the Invention
The present invention relates to non-aqueous electrolyte secondary batteries.
2. Description of Related Art
In recent years a non-aqueous electrolyte secondary battery has drawn attention as a high energy density battery. The non-aqueous electrolyte secondary battery comprises a negative electrode active material employing a metallic lithium, or an alloy or a carbon material that is capable of intercalating and deintercalating lithium ions, and a positive electrode material employing a lithium-containing transition metal oxide represented by the chemical formula LiMO2 (where M is a transition metal). For the electrolyte solution, cyclic carbonates such as ethylene carbonate and propylene carbonate, cyclic esters such as y-butyrolactone, and chain carbonates such as dimethyl carbonate and ethyl methyl carbonate, are used either alone or in combination.
A representative example of the lithium-containing transition metal oxide used for the positive electrode is lithium cobalt oxide (LiCoO2), which has already been in commercial use as a positive electrode active material for non-aqueous electrolyte secondary batteries. A problem with the use of the lithium-containing transition metal oxide having a layered structure, such as represented by lithium cobalt oxide, alone as the positive electrode active material, however, is that, because the positive electrode active material undergoes change in volume associated with a charge-discharge process, capacity degradation occurs as the charge-discharge process is repeated; that is, cycle performance deteriorates.
Japanese Unexamined Patent Publication Nos. 2003-7299 and 2003-331846, for example, propose that, in order to improve cycle performance of the positive electrode active material, lithium cobalt oxide is used for the positive electrode, and the surface thereof is treated with MXOk, especially with an aluminum phosphate compound represented by AlPOk. Although the treatment of the surface of lithium cobalt oxide with an aluminum phosphate improves cycle performance, a problem is that the initial efficiency of the positive electrode active material degrades.
In addition, many attempts have been made to improve cycle performance by addition of another element to the positive electrode active material, or substitution therewith, and one example of the attempts is the addition of zirconium, magnesium, or the like to the positive electrode active material (cf. Japanese Patent No. 3045998). However, the addition of another element to the lithium-containing transition metal oxide also causes the problem of the degradation in initial efficiency.
In recent years, the demand for batteries with higher energy density has been growing greatly, and accordingly, materials having a greater initial charge-discharge efficiency are desired for both the positive electrode active material and the negative electrode active material. Since the techniques proposed in the foregoing patent publications bring about the degradation in initial efficiency of the positive electrode active material, the techniques are undesirable from the standpoint of attaining higher energy density batteries. In particular, when an active material with a high initial charge-discharge efficiency such as graphite is used as the negative electrode active material, the initial efficiency of the positive electrode has a great influence on the energy density of the battery, and an improvement in the initial efficiency has been desired as well as improvements in various characteristics of the positive electrode.